Low energy electron microscopy (LEEM) and photo electron emission microscopy (PEEM) are both examples of cathode lens microscopy, in which a strong electric field is maintained between a sample under study and an objective lens of a microscope. In such instruments, the sample is considered the cathode and the objective lens is considered the anode. Electrons are reflected from the sample in the case of a LEEM instrument, or photo-emitted by the sample in the case of a PEEM instrument, at low energy, for example, less than 500 eV. The electrons are accelerated into the objective lens, reaching an energy of 10-30 keV. Subsequently, these electrons are utilized to form an image of the sample on a viewing screen.
The backfocal plane of the objective lens of the microscopy instrument provides an image of the angular distribution of the electrons, which contains information on the arrangement of the atoms in the outer layers of the sample. This image is considered a low energy electron diffraction (LEED) pattern for LEEM, or a photo electron diffraction (PED) pattern for PEEM. The energy distribution of these electrons may also contain information about the electronic and chemical nature of the surface under study.
Energy filtering of the electrons allows an operator to view an image of the sample at a specified electron energy corresponding to, for example, the binding energy of electrons of a particular chemical element. Alternatively, by operating projector and spectrometer lenses of the microscope at a different excitation, the energy filtered PED pattern may be observed. The operator may choose to record an energy spectrum of the photo emitted electrons. The combination of an energy filtering cathode lens microscopy instrument with synchrotron radiation provides the operator with an extremely powerful analytical tool in the study of surface and interface structure and composition.
Aberration-corrected energy-filtered LEEM/PEEM has been pursued by several research groups. In general, the experimental approaches taken are quite complex, and include, for example, a dispersion-free prism array as outlined by Rose and Preikszas for the German SMART project, as well as the Berkeley-based PEEM project. In such an approach, energy filtering is accomplished through the inclusion of an omega energy filter in the projection column. Aberration correction is accomplished through the inclusion of an electron mirror on one of the four faces of the dispersion-free prism array. The dispersion-free prism array, the electron mirror, and the omega filter are electron optical components of high complexity. Combining all three elements in a single microscopy instrument has proven to be a non-trivial exercise in instrument design and construction. Therefore, it is desirable to achieve a novel instrument geometry that relies on a simpler prism array allowing dispersion, and which allows for a simplified incorporation of energy-filtering and aberration-correcting functions.